1. Field of the Invention
This invention relates to an injector mounting structure for engines, adapted to fix injectors in a cylinder head of an engine, especially, a direct injection type engine.
2. Description of the Prior Art
In recent years, a DOHC engine having two cam shafts per cylinder and two suction valves or exhaust valves on each cam shaft, and a multi-valve engine have been becoming leading engines for meeting the requirements for improving the suction and exhaust efficiencies of engines, increasing the rotational speed and an output level thereof and minimizing the fuel consumption thereof. In such an engine, injectors are provided in a cylinder head so that one injector is opposed to a substantially central portion of one cylinder, and also two parallel-arranged cam shafts so as to hold one injector therebetween, each injector being positioned in the center of a region surrounded by two suction valves or exhaust valves provided on each cam shaft. An example of an engine having injectors arranged in this manner is disclosed in Japanese Patent Laid-Open No. 101018/1992.
In the engine disclosed in this publication, a mechanism for operating suction valves and exhaust valves comprises a cam rotated by a cam shaft, and a rocker arm contacting the cam and converting the action of the cam into reciprocating movements of the suction and exhaust valves. Therefore, a large space is required above a cylinder head to hold the valve operating mechanism therein, and this causes the dimensions of the engine to increase. A bracket is prepared for each cam shaft, and fixed to the cylinder head by two bolts so as to retain the cam shaft. The fixing of each injector to the cylinder head is also done by using other bolts. Consequently, the number of parts used in conjunction with the injectors and cams, such as brackets and bolts for fixing the same and injector fixing bolts increases to cause the cost of the engine to increase.
The applicant of the present invention proposed (refer to Japanese Patent Laid-Open No. 254167/1996) a structure capable of rationally utilizing a space above a cylinder head in a DOHC multi-valve direct injection type engine having a center nozzle structure in which injectors are provided in the centers of regions surrounded by suction and exhaust valves. This structure is characterized in that a cam bracket fixing bolt and an injector clamp member fixing bolt are united with each other, and this can reduce a space required to fix the injectors. According to this structure, a cam shaft can be fixed in a position lower than that in the structure disclosed in Japanese Patent Laid-Open No. 101018/1992, and the opening and closing of the suction and exhaust valves are done by using cams which directly work on these valves, so that rocker arms may not be used. This enables the space saving to be effected above the cylinder head, and the number of parts to be reduced.
In this direct injection type DOHC 4-valve diesel engine, these various advantages are produced but the following problems remain in a structure for fixing injectors to the cylinder head by clamps.
(1) Since a cam bracket is disposed on an imaginary line passing the center of a cylinder and orthogonally crossing cam shafts, it is difficult to reduce the distance between the axes of the two cam shafts. A cam bracket fixing bolt and an injector clamp member fixing bolt are united with each other but, if the cam bracket is made as a single part to be used in common with the two cam shafts, it interferes with the relative injector. Therefore, the cam shaft is provided separately on each cam shaft. Under the circumstances, two fixing bolts are still necessary for setting each bracket, and also two fixing bolts for securing a cam bracket to the cylinder head are still required to be provided between two cam shafts. As a result, the reduction of an included angle between a suction and exhaust valve is restricted, and it is under apprehension that a required compression ratio is secured when this structure is applied to a diesel engine. If the suction/exhaust valve included angle is set close to zero forcibly, the diameters of the ports of the suction and exhaust valves necessarily become smaller to cause the volumeric efficiency of the engine to decrease, and a pumping loss to increase. Since the distance between the axes of the cam shafts cannot be reduced, a correspondingly wide space is required above the cylinder head, so that it is difficult to meet the demand for forming the structure more compactly.
(2) Each cam shaft is fixed by a separately provided independent cam bracket, and each injector to a cylinder head by a single clamp member. Four bolts are required for each cam bracket and each clamp member. Accordingly, the number of parts is still large, and reducing the cost is difficult.